With 99.4 per cent of votes tallied in the presidential runoff, Milei had 55.7 per cent and Economy Minister Sergio Massa 44.3 per cent, according to Argentina's electoral authority.

It is the widest victory margin in a presidential race since the South American country's return to democracy in 1983.

In the streets of Buenos Aires, drivers honked their horns and many took to the streets to celebrate in several neighbourhoods. Outside Milei's party headquarters, a hotel in downtown Buenos Aires, a full-on party kicked off with supporters singing, buying beers from vendors and setting off coloured smoke bombs.

They waved Argentine flags and the yellow Gadsden flag, emblazoned with the words “Don't Tread On Me”, which Milei's movement has adopted.

Inside, the self-described anarcho-capitalist who has been compared to former US President Donald Trump, delivered his victory speech, saying the “reconstruction of Argentina begins today”.

“Argentina's situation is critical. The changes our country needs are drastic. There is no room for gradualism, no room for lukewarm measures,” Milei told supporters, who chanted “Liberty, liberty!” and “Let them all leave” in a reference to the country's political class.

Massa of the ruling Peronist party had already conceded defeat, saying Argentines “chose another path”.

“Starting tomorrow ... guaranteeing the political, social and economic functions is the responsibility of the new president. I hope he does,” Massa said.

With a Milei victory, the country will take an abrupt shift rightward and a freshman lawmaker who got his start as a television talking head blasting what he called the “political caste” will assume the presidency.

Inflation has soared above 140 per cent and poverty has worsened while Massa has held his post. Milei has said he would slash the size of the government, dollarise the economy and eliminate the Central Bank as a way to tackle galloping inflation that he blames on successive governments printing money indiscriminately in order to fund public spending. He also espouses several conservative social policies, including an opposition to sex education in schools and abortion, which Argentina's Congress legalised in 2020.

“This is a triumph that is less due to Milei and his peculiarities and particularities and more to the demand for change,” said Lucas Romero, the head of Synopsis, a local political consulting firm. "What is being expressed at the polls is the weariness, the fatigue, the protest vote of the majority of Argentines.”

Massa's campaign cautioned Argentines that his libertarian opponent's plan to eliminate key ministries and otherwise sharply curtail the state would threaten public services, including health and education, and welfare programmes many rely on.

Massa also drew attention to his opponent's often aggressive rhetoric and openly questioned his mental acuity; ahead of the first round, Milei sometimes carried a revving chainsaw at rallies.

“There were lot of voters that weren't convinced to vote Milei, who would vote no or blank. But come the day of the vote, they voted for Milei because they're all pissed off,” Andrei Roman, CEO of Brazil-based pollster Atlas Intel, said by phone.

“Everyone talked about the fear of Milei winning. I think this was a fear of Massa winning and economy continuing the way it is, inflation and all that.”

Milei accused Massa and his allies of running a “campaign of fear” and he walked back some of his most controversial proposals, such as loosening gun control. In his final campaign ad, Milei looks at the camera and assures voters he has no plans to privatize education or health care.

Milei's screeds resonated widely with Argentines angered by their struggle to make ends meet, particularly young men.

“Incredibly happy, ecstatic, it's a global historical phenomenon!” Luca Rodriguez, a 20-year-old law student, said outside Milei's headquarters after spraying a bottle of champagne into the air onto those around him, who squealed with glee.

“I want to break free from this ridiculous elite that takes away all our rights, all the tax money that pressures us and doesn't let us live in peace.”

Two Milei supporters in the raucous crowd were 32-year-old identical twins, both dressed in matching grey tank tops with Argentine flags draped over their shoulders.

“We want a change, we want everything to improve,” Amilcar Rollo said beside his brother, Gabriel. “It's the hope for something new from someone who hasn't been there and has different ideas. Otherwise, it's just the same as always.”

Most pre-election polls, which have been notoriously wrong at every step of this year's campaign, showed a statistical tie between the two candidates or Milei slightly ahead.

Underscoring the bitter division this campaign has brought to the fore, Milei received both jeers and cheers on Friday night at the legendary Colon Theatre in Buenos Aires.

The acrimony was also evident Sunday when Milei's running mate, Victoria Villaruel, went to vote and was met by protesters angry at her claims that the number of victims from Argentina's bloody 1976-1983 military dictatorship is far below what human rights organizations have long claimed, among other controversial positions.

The vote took place amid Milei's allegations of possible electoral fraud, reminiscent of those from Trump and former far-right Brazilian President Jair Bolsonaro. Without providing evidence, Milei claimed that the first round of the presidential election was plagued by irregularities that affected the result.

Experts say such irregularities cannot swing an election, and that his assertions were partly aimed at firing up his base and motivating his supporters to become monitors of voting stations. Many have expressed concerns they undermine democratic norms.

Both Bolsonaro and Trump congratulated Milei on social media.

“The whole world was watching! I am very proud of you,” Trump wrote on his platform, Truth Social. “You will turn your Country around and truly Make Argentina Great Again!”

And posting on X, formerly Twitter, U.S. Secretary of State Antony Blinken also commended Milei.

“We look forward to continuing bilateral cooperation based on shared values and interests,” Blinken wrote.